In respirator treatment, a patient is connected to a respirator, which aids the patient in breathing. The respirator typically comprise a means of mixing and forming a breathing gas having a predetermined ratio of one or more gases, the pressurized sources for which are connected to the respirator. The gas mixture has to contain a sufficient amount of oxygen. For this reason, one of the gases is always O.sub.2, or alternatively, in the most simplified one gas source devices, is air. The other gas or gases to be mixed with O.sub.2 comprise most typically air, N.sub.2 O, and sometimes also He or Xe.
To perform the mixing function, each of the gas flow paths has a regulating means, typically a valve, to regulate the gas flow. In current state of the art, these regulating means are driven by a microprocessor control unit according to the information the control unit receives from various pressure, flow, and/or position sensors to regulate to predetermined control parameter values.
The control parameters include a plurality of various criteria defining the respiration pattern. These parameters are e.g. inspiration and expiration times, respiration rate, tidal volume (the volume of one breath), inspiratory flow, inspiration pressure, and positive end expiratory pressure. Modern respirator treatment encourages the patient to breathe by himself. State of the art respirators are thus equipped to both support the spontaneous breath trials by the patient and to provide pressure to assist or carry out breathing when necessary.
The patient is connected to the respirator through a breathing circuit comprising an inspiratory limb, expiratory limb, and a patient limb terminating in an endotracheal tube or breathing mask. These elements are connected together at a Y-piece connector. The inspiratory limb is connected to the respirator at the outlet for the breathing gas mixture and to the inlet of the Y-piece connector. The expiratory limb is connected to the outlet of the Y-piece connector and to a expiration valve, normally also included in the respirator. The patient is connected to the breathing circuit through the patient limb to the endotracheal tube or breathing mask.
When the patient is inspiring, the expiration valve is closed. The breathing gas is supplied with overpressure through the inspiratory limb to endotracheal tube or breathing mask and further to the lungs of the patient. During expiration, an inspiratory valve also included in the respirator is closed, and the expiration valve is opened, releasing the pressure within the lungs. This relief is based on the tension and elasticity of the lungs.
Tidal volumes range from a few tens of milliliters for the smallest babies to more than one liter for adults. The respiration rates also vary from tens/minute down to a few/minute. Typical breathing gas flow rates extend from the level of one liter/min up to 10 liters/min, but may even exceed these. The volume of breathing gas is delivered during the inspiration phase, representing typically one third of the respiration cycle. Thus the peak inspiratory flow may easily exceed 30 liters/min and reach momentarily 100 liters/min in inspiratory regulation based on a preset inspiration pressure.
During respirator treatment, for diagnostic and therapeutic purposes, a need for completing the breathing gas with a special gas exists. Typical special gases are nitric oxide (NO) for improvement of lung perfusion and thus patient O.sub.2 uptake raising the blood oxygen saturation, SF.sub.6 (sulfur hexa fluoride) for measuring the lung functional residual volume (FRC) and nitrous oxide (N.sub.2 O) for measuring the lung capillary blood flow. The gases may even be combined into a single gas reservoir for multiple action behavior as shown e.g. in EP 640357.
A special problem arises from the supply of, or dosing, NO due to the extremely small amounts of gas to be regulated. The usual levels start from 0.1 ppm (part per million) up to some tens of ppm. To facilitate the regulation, the NO gas is diluted to a ratio about 100 ppm-1000 ppm in N.sub.2 in the pressure container from which the gas is delivered. Another problem is the reactivity of the NO. Together with O.sub.2 an extremely toxic end product, nitrogen dioxide (NO.sub.2), will be produced from the NO. To minimize this production, it is advantageous to design the delivery system to minimize the exposure time the NO and O.sub.2 have to react with each other.
A further requirement for an advantageous form of the delivery system comes from user ergonomy. The personnel taking care of the patient often work near the mouth of the patient and with the above described breathing circuit. The less additional equipment required in this area, the better the equipment will be from the ergonometric standpoint.
State of the art technology includes various delivery systems to deliver the special gas in preset amounts through the breathing circuitry to the patient. The delivery systems are fitted with the respiratory breath circuitry. The delivery system may operate either continuously or synchronously with inspiration. EP 640356 presents a continuous flow delivery system for spontaneously breathing patients. In this system the special gas is mixed with the breathing gas in the gas mixer. The breathing circuit is modified with a connecting tube. Both the inspiratory and expiratory limbs are connected to this connecting tube. Thus, the patient inhales the gas through the inspiratory limb from the connecting tube and exhales through the expiratory limb to the connection tube. The mixer delivers a continuous flow of breathing gas with an added, predetermined concentration of the special gas, to the connecting tube. In the delivery system, the continuous flow is set to 20 liters/min to fulfill peak flow requirements. Even larger peak flows are possible as the backflow from the connecting tube. The relatively high flow rate is used to minimize the reaction time of NO and O.sub.2 and to minimize the risk of rebreathing the gas the patient has already expired through the connecting tube. The arrangement solves the forementioned problems of the NO delivery, but due to the high flow, this arrangement causes a very large loss of breathing gas and NO. Particularly, the NO may be very expensive causing also economic losses. Due to the toxicity of NO and the end product NO.sub.2, an evacuation system is a prerequisite for safe operation. Further, this arrangement does not solve the delivery problems with patients requiring breathing aid from respirator.
A delivery system in connection with a respirator is shown in EP 640357. The system described is for delivery of special gas, which in this case is a mixture of NO, a tracer gas, and diluent gas, in constant concentration, through the breathing circuit into lungs. The special gas is delivered into the breathing circuit via a connecting tube. The delivery system is feedback controlled through a tracer gas measured from the expiratory end of the respirator. As the delivery system is controlled by the ventilator, although not so described, one can, thus, conclude that the special gas delivery is pulsatile in nature and synchronous with inspiration flow variations. To minimize the risk for NO.sub.2 formation, it is presented as advantageous to lead the special gas mixture connecting tube as close to the patient lungs as possible, even through the endotracheal tube to inside the lungs.
However, the problem of small flows of the special gas remains and is even emphasized. Firstly, the longer the connection tube is, the longer time it will take before the special gas will reach the patient end of the tube when starting the system. An example of a small delivery could be as follows. A 0.5 ppm NO concentration from 1000 ppm source, in a tidal volume of 50 ml is to be delivered in one second. This requires a pulse volume of 0.025 ml. For a connecting tube having diameter 1 mm, this volume will occupy 3 cm in the tube. Secondly, and even worse, during inspiration, when the special gas should be delivered, the pressure within the breathing circuit will increase. The increasing pressure will cause gas compression in the connecting tube preventing the small special gas flow into lungs. During expiration the pressure is relieved and the compressed special gas flows out from the connecting tube directly into the expiration flow and the required therapeutic effect is not achieved. Both of these problems are made worse the smaller the special gas flow is and the longer the connecting tube is. Thirdly, the closer the connection tube or the second gas mixer is to the patient, the worse the solution becomes ergonomically. In another embodiment of EP 640357, the special gas is mixed within the respirator. The NO.sub.2 formation may however be increased due to the prolonged reaction time between the special gas and the breathing gas described in an article written by R. Kuhlen et al. entitled "Nitrogen dioxide (NO.sub.2) production for different doses of inhaled nitric oxide (NO) during mechanical ventilation with different tidal volumes using the prototypes for the administration of NO. " The harmful composites may however be removed from the breathing gas immediately before inhalation by scrubbers as presented in U.S. Pat. No. 5,485,827. Also, some wastage of special gas takes place, since all the breathing gas leaving the respirator does not reach the lungs. Thus the arrangement causes a need for evacuation of the gas exhaled from the respirator. From the point of view of ergonometry, this embodiment is advantageous since no additional equipment near the patient is required. However, the possible need of scrubbers will impair the ergonomy.
U.S. Pat. No. 5,423,313 describes a special gas delivery arrangement to be used in connection with respiratory treatment apparatus comprising similar elements to that of EP 640357. This arrangement differs from EP 640357 from the point of view of the control. Whereas EP 640357 targets for constant concentration of the special gas in the inspired mixture, the system described in U.S. Pat No. 5,423,313 delivers the special gas in pulses independently of the respiratory breath cycle. An advantageous pulse frequency is defined to be 53 Hz. It is claimed that with the high frequency pulses, more even gas distribution is reached in the lungs due to diffusion. However, the high frequency pulses do not change the fact that the total special gas flow range may be very low, and as such, the problems listed for EP 640357 also exist here.
A further NO delivery system is presented in EP 659445. This document also describes an arrangement designed for delivering a constant NO concentration into the inspired breathing gas. It is characteristic of this arrangement that the device can be used with respirator treatment but is not bound to that. A breathing gas flow sensor is included in the equipment. From this sensor, the control unit receives data to regulate the NO gas flow to maintain the required breathing gas concentration. The NO gas is derived from an NO/N.sub.2 mixture gas reservoir containing 1000 ppm NO. In the event such a low concentration of NO is required that the system is otherwise unable to reduce the concentration to the desired point, a further equipment in the form of an N.sub.2 pressure source, regulating valve, and NO concentration analyzer are provided to further dilute the NO mixture concentration, thus adding gas volume to the NO mixture flow to be regulated. The increasing flow decreases also the described problems caused by the ventilatory pressure variations and the outlet channel volume filling, but does not eliminate same.